Was angered by MP in Slough (5)
I believe the answer is:
swamp
'slough' is the definition.
(I've seen this before)
'was angered by mp' is the wordplay.
I cannot really understand how this works, but
an anagram of 'was' is 'swa' which is present in the answer.
'mp' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'in' is the link.
Can you help me to learn more?
(Other definitions for swamp that I've seen before include "Bog, marsh" , "Bog; inundate" , "In flood" , "Engulf" , "Overwhelm; wetland" .)